A common form of electronic counter is a digital incremental divide-by-10 (decade) circuit commonly comprising a set of four or five D-type filp-flops which increment with the application of input pulses, such as at 100 meter intervals of travel of a motor vehicle, together with a logical function to detect the presence of the "nine" state which acts to clear the decade to zero on receipt of the next input pulse, and sends a "carry" pulse to the subsequent decade or decades.
One such counting circuit is described in U.S. Pat. No. 4,663,770. The counter described in the specification of that patent application incorporates an array of D-type flip-flops forming a plurality of decade counters. Each flip-flop incorporates a master flip-flop and a slave flip-flop in order to avoid problems associated with the synchronization of the clock signal with a rapidly changing data input signal.
Counters of this type are generally manufactured as custom made integrated circuits and it would be desirable to reduce the area occupied by a single counter integrated circuit so that more such circuits can be fitted onto a single silicon wafer and their unit cost can be reduced. It would also be advantageous to simplify the testing of a single circuit, again reducing the unit cost of manufacture.